While Powdery Mildew will not usually kill a plant, it does distort growth, reduce flowering, discolor foliage and impact vigor. Not desirable for garden or nursery.
Before treating for Powdery Mildew, make sure that you have properly identified the disease.
Powdery Mildew is commonly confused with Downy Mildew. While they are two very different fungi, I understand why they are often misidentified: They both have "mildew" in their names and they are characterized by a white growth on the leaves. In addition, Powdery and Downy Mildews tend to occur in the spring and fall when temperatures are lower and humidity is high.
HOW TO IDENTIFY
If you think you have Powdery Mildew, look to see which side of the leaf is covered with the white growth. If it is on the upper side of the leaf, it is probably Powdery Mildew (Downy Mildew sporulates on the underside of the leaf).
Then look at the growth pattern. Powdery Mildew has diffuse margins while Downy Mildew creates angular leaf spots. This is illustrated in the image of the Japanese Magnolia below:
The white growth is the reproductive phase of the fungus, Oidium. The vegetative portion of the fungus is actually inside the leaf, taking nutrients directly from the leaf cells. The spores only appear when it is cool and humid.
See how the Dogwood leaf is purpling in the areas where there are spores on the surface? Those are the areas where the fungus has taken nutrients from the leaf. The premature fall coloration occurs because these areas of the leaf are not photosynthesizing properly.
MANAGING POWDERY MILDEW
The best way to prevent Powdery Mildew is to use resistant species and cultivars. Resistance has been bred into certain Crape Myrtles, Hydrangeas, Phlox and other plants.
If that's not an option and you have an infection:
- Apply lightweight paraffinic oil as a coating to the leaves to provide good management. Spray the leaves on a weekly basis (when daily temperatures are below 80F) with a 1-2% solution.
- Increase air circulation in the growing area to reduce the humidity level.
- Grow where the plant gets at least partial sun during the day.
- Rake up fallen leaves during the dormant season. Powdery Mildew overwinters as cleistothecia (survival spores) on these fallen leaves and will reinfect plants the following spring when the spores splash back onto the plants.
The powdery growth is made of these chains of round spores. These spores (conidia) are spread to new leaves and plants by air currents and the wind.*Above image from Cornell University Department of Plant Pathology.